The present invention relates to an exhaust gas recirculation system (EGR system) which reduces Nitrogen Oxide emissions in the exhaust gas of a motor vehicle.
An EGR system is known which recirculates a part of the exhaust gases into the intake system of the engine so as to supply inert gases into the air/fuel mixture, for the purpose of lowering the combustion speed and the maximum combustion temperature of the air/fuel mixture, thereby reducing Nitrogen Oxide emissions in the exhaust gas. Two types of such an EGR system are known; one is an ON-OFF type EGR system and the other is a back pressure controlling type EGR system.
In the ON-OFF type EGR system, a two-position control EGR valve is arranged on the EGR pipe which interconnects the exhaust manifold and the intake manifold. When the engine starts, the EGR valve is opened and a part of the exhaust gas is sucked into the intake manifold by the suction force of the vacuum pressure in the intake manifold. When the engine is stopped, the EGR valve is closed and the EGR system does not operate. In this type of EGR system, however, under a light load condition of the engine the EGR ratio increases, especially when the number of revolutions of the engine is low. Accordingly, the output force of the engine is lowered and it becomes more difficult to obtain satisfactory engine performance.
In order to mitigate the above mentioned drawbacks in the ON-OFF type EGR system, a back pressure controlling type EGR system is known in which the EGR ratio is maintained substantially constant relative to the number of revolutions of the engine. This known back pressure controlling type EGR system comprises:
A diaphragm type EGR valve mounted on an EGR pipe which connects the exhaust manifold and the intake manifold; PA1 An opening path for atmosphere diverged from a connecting pipe which connects the diaphragm chamber of said EGR valve and the EGR port near the carburetor throttle valve; and PA1 A diaphragm type vacuum control valve (VCV) arranged at the opening end of said opening path, the diaphragm chamber of said VCV communicating with the inlet of said EGR valve. However, in this type of EGR system, especially under a light load condition of the engine, the above mentioned drawbacks such as the output force of the engine being lowered are not acceptably obviated.